For many years, the only clear physiological role of erythropoietin had been its control of the production of red blood cells. Recently, several lines of evidence suggest that erythropoietin, as a member of the cytokine superfamily, performs other important physiologic functions which are mediated through interaction with the erythropoietin receptor (erythropoietin-R). These actions include mitogenesis, modulation of calcium influx into smooth muscle cells and neural cells, and effects on intermediary metabolism. It is believed that erythropoietin provides compensatory responses that serve to improve hypoxic cellular microenvironment as well as modulate programmed cell death caused by metabolic stress. Although studies have established that erythropoietin injected intracranially protects neurons against hypoxic neuronal injury, intracranial administration is an impractical and unacceptable route of administration for therapeutic use, particularly for normal individuals. Furthermore, previous studies of anemic patients given erythropoietin have concluded that peripherally-administered erythropoietin is not transported into the brain (Marti et al., 1997, Kidney Int. 51:416-8; Juul et al., 1999, Pediatr. Res. 46:543-547; Buemi et al., 2000, Nephrol. Dial. Transplant. 15:422-433.).
Various modified forms of erythropoietin have been described with activities directed towards improving the erythropoietic activity of the molecule, such as those with altered amino acids at the carboxy terminus described in U.S. Pat. No. 5,457,089 and in U.S. Pat. No. 4,835,260; erythropoietin isoforms with various numbers of sialic acid residues per molecule, such as described in U.S. Pat. No. 5,856,298; polypeptides described in U.S. Pat. No. 4,703,008; agonists described in U.S. Pat. No. 5,767,078; peptides which bind to the erythropoietin receptor as described in U.S. Pat. Nos. 5,773,569 and 5,830,851; and small-molecule mimetics as described in U.S. Pat. No. 5,835,382.
It is towards the use of an erythropoietin for protecting, maintaining, enhancing, or restoring erythropoietin-responsive cells and associated cells, tissues and organs in situ as well as ex vivo, and to delivery of an erythropoietin across an endothelial cell barrier for the purpose of protecting and enhancing erythropoietin-responsive cells and associated cells, tissues and organs distal to the vasculature, or to carry associated molecules, that the present invention is directed.